Semiconductor materials of the Group II-VI elements and especially those containing mercury, cadmium, and tellurium are useful as photodetectors for infrared radiation. Applications include heat sensors in automobile engines, solar cells, sensors for road obstacles, and infrared detectors for night vision and heat-seeking missiles.
In order to take advantage of the electronic properties for the II-VI materials, devices based on heterojunction diodes and superlattices must be fabricated. Device fabrication requires the ability to grow abrupt junctions over large areas but this has been hampered by the high temperatures required to grow tellurium containing alloys. The high growth temperatures coupled with low growth rates result in diffusion of mercury across the heterojunction, thus destroying its integrity and usefulness.
The apparent solution is to lower the growth temperatures and increase the growth rates. Metalorganic chemical vapor deposition (MOCVD) is a technique well-suited to the deposition of II-VI semiconductors. MOCVD is commonly employed in the semiconductor, optical, and optoelectronic industries for doping or coating a suitable substrate. MOCVD essentially involves depositing a thin film of an element or compound onto a substrate such as silicon. The deposited films can be sources of doping impurities which are driven into the substrate or the films themselves can have different electrical or optical properties than the substrate. The properties of the film depend on the deposition conditions and the chemical identity of the deposited film. MOCVD has significant advantages in that the process is uniform and amenable to scale-up. New source reagents and growth techniques that permit junction fabrication at low temperatures are required.
MOCVD of tellurium containing alloys is limited by the availability of appropriate tellurium source reagents. The most important criteria for effective reagents in low temperature CVD of tellurium are:
1) volatility of the tellurium source reagent, PA1 2) efficient decomposition of the source reagent to elemental tellurium without incorporation of impurities into the product film, and PA1 3) chemical and elemental purity of the source reagent.
The ideal reagent for CVD of a metal or metalloid element would be characterized by high volatility (vapor pressure of .gtoreq.10 mm Hg at room temperature) and rapid low temperature decomposition to the elemental form (e.g., decomposition at temperatures of about 200.degree. C. versus temperatures approaching 500.degree. C., which are required for the presently available tellurium source reagents) with no codeposition of carbon or other contaminants into the film of the desired element. Dimethyl and diethyltellurium are currently utilized as source reagents for the CVD of tellurium and are commercially available. Neither of these reagents possesses labile Te-C bonds and both require high temperatures (&gt;450.degree. C.) for successful deposition of tellurium containing alloys (Hoke, W. E.; Lemonias, P. J.; Korenstein, R., J. Mater. Res., 1988, 3, 329).
No currently available tellurium source reagents are optimum for the intended applications. New tellurium reagents with weaker tellurium-carbon bonds have been introduced since 1985 and are included in the table below. The newer reagents have secondary or tertiary alkyl or allyl substituents, since it has been reasoned that these substituents will lead to enhanced decomposition at lower temperatures. While improvement is seen in the decomposition temperature, volatility of the starting reagent is sacrificed.
______________________________________ Organotellurium Source Reagents Vapor Pressure Growth Temp. (.degree.C.) (mm/.degree.C.) ______________________________________ Dimethyltellurium 450 51/25 Diethytellurium 450 9.3/25 Diisopropyltellurium 390 5.6/30 Ditertbutyltellurium 320 4/40 Diallyltellurium (280) 3.5/45 Methyl(allyl)tellurium 290 8/30 2,5-dihydrotellurophene 250 -- Dimethylditellurium (250) 0.26/23 ______________________________________ Hoke, W. E.; Lemonias, P., Appl. Phys. Lett., 1985, 46, 398. Kisker, D. W.; Steigerwald, M. L.; Kometani, T. Y.; Jeffers, K. S., Appl. Phys. Lett., 1987, 50, 1681. Parsons, J. D.; Lichtmann, L. S.; J. Cryst. Growth 1988, 86, 222. Parsons, J. D.; Lichtmann, L. S.; J. Cryst. Growth, 1988, 86, 217.
Perfluoroalkyltellurium compounds represent a novel alternative to the materials currently in use, yet until now these reagents have not been studied under MOCVD conditions, nor in many cases even synthesized. The increased volatility for alkyl(perfluoroalkyl)tellurium compounds, without significant alteration of growth temperatures or film quality, renders these compounds potentially superior source reagents for MOCVD.
The absence of intermolecular forces in perfluoroalkyl compounds often leads to increased volatility. Boiling points for a number of main group alkyl compounds are compared with the corresponding perfluoro alkyl compounds below and demonstrate the trend toward lower boiling points for the fluorocarbon derivatives.
__________________________________________________________________________ Boiling Points of Selected Organometallic Compounds at 1 atm Compound (CH.sub.3).sub.2 S (CF.sub.3).sub.2 S (CH.sub.3).sub.2 Se (CF.sub.3).sub.2 Se (CH.sub.3).sub.2 Te (CF.sub.3).sub.2 Te __________________________________________________________________________ b.p. (.degree.C.) 37.3 -22 54.5 -- 82 23 __________________________________________________________________________ CRC Handbook of Chemistry and Physics 55th Ed. 1974-75; Herberg, S.; Naumann, D., Z. Anorg. Allg. Chem., 1982, 492, 95
A further concern that arises in MOCVD of tellurium-containing materials is the high toxicity of hydrogen telluride or the dialkyltellurium compounds. In addition, these materials possess odors that are so foul and persistent that they are exceedingly unpleasant and inconvenient to work with. The excursion of even extremely small amounts of the lower alkyl tellurides into the air creates an intolerable working environment. Thus new organometallic compounds of tellurium, with enhanced volatility and reduced toxicity and stench factors, would be desirable.